The invention is based on a pilot control valve for a fuel injection valve.
Fuel injection valves conventionally have a control chamber, which communicates constantly, via a throttle, with a high-pressure fuel source. If the control pressure prevailing in the control chamber is high, then a valve member of the fuel injection valve is kept in the closing position. The control chamber can be relieved via a second throttle, which is controlled by a pilot control valve. As soon as the pilot control valve opens the second throttle, the control chamber is relieved, and the valve member changes to the opening position, so that the injection can take place. When the pilot control valve closes the second throttle again, the valve member is returned to the closing position as a result of the pressure increase in the control chamber. For the quality of fuel injection, the speed, precision and replicability of the opening and closing motions of the pilot control valve are of decisive importance.
Pilot control valves that have these properties to a satisfactory extent or that make it possible to establish them have a complicated design, comprising many parts, and thus they can be assembled and adjusted only by expending a great deal of time.
Thus the known pilot control valve shown in FIG. 4 includes two housing parts 1 and 2, of which the second part 2, is inserted into a well-like recess in the first part 1. A union nut 33 keeps the two housing parts pressed against one another in the direction of a longitudinal axis z-z of the pilot control valve. On the bottom of the recess there is a threaded bore, on the bottom of which the valve seat 3 of the pilot control valve is disposed. Via two throttles 4 and 5 and the intervening control chamber 6 of the injection valve, the valve seat communicates with an inlet neck 7 for fuel from a high-pressure pump.
A valve closing member 9 is displaceably guided by an anchor disk 8 between a closed position in which the valve closing member 9 rests on the valve seat 3, and an open position; the stroke between these two positions is determined by the thickness of a spacer disk 10, which is clamped between the bottom of the threaded bore and the anchor disk.
A magnet core 11 is let into the second housing 2; it contains a coil 12 for generating a magnetic field. In the chamber 13 defined jointly by the two housing parts 1, 2, an armature plate 14 firmly joined to the valve closing member 9 is disposed facing towards the magnet core 11. In both the open and the closed state of the pilot control valve, there should be an air gap between the armature plate and the magnet core. The width of this gap is determined by the thickness of a second spacer disk 15, which is disposed between the walls of the second housing part and the bottom of the recess in the first housing part. The spacer disk 15 bears the contact pressure of the union nut 33 and must therefore not be elastic or yielding.
It is difficult to manufacture all the parts of the known pilot control valve on a mass-production basis in such a way that the finished pilot control valves replicably have a desired switching performance. Readjusting a pilot control valve that lacks the desired switching performance is very complicated, since this requires dismantling the valve again, and the spacer disks 10, 15 might have to be replaced.